(1) Field of the Invention
The present invention relates generally to an input circuitry of a fork lift truck control system using a microcomputer, and more particularly to a circuit for detecting and signalling to the microcomputer through a common bus the height of a fork mounted on an upright above its lowest position based on the length of a chain passed through a chain wheel attached on the top end of an inner mast of the upright.
(2) Description of the Prior Art
A fork lift truck comprises, in general, a load lifting mechanism and vehicle body. The load lifting mechanism comprises a vertically elongated guide rail called an "upright" and a fork slidable along the upright.
The load lifting mechanism further comprises: (a) a tilt cylinder attached to a front portion of the vehicle body having a piston interlinked with an outer mast constituting the upright rotatably supported by the front portion of the vehicle body, so that a tilting angle of the upright with its vertical position with respect to the ground as a neutral position can be adjusted; (b) a lift cylinder attached along the elongated direction of the outer mast having a piston interlinked with an inner mast constituting the upright upwardly extendable from the outer mast; (c) a chain wheel rotatably attached to the top end of the piston of the lift cylinder and engaged with a chain, one end of the chain attached to the outer mast or lift cylinder body and the other end of the chain attached to either a lifting member fitted into the inner mast so as to move upward and downward together with the inner mast and fork or to the fork engaged with the lifting member, so that the movement of the lift cylinder causes the inner mast to elevate upward and accordingly causes the fork to move upward along the outer mast by means of the chain engaged with the chain wheel, thereby lifting a load piled thereon.
A microcomputer system has been proposed which performs an automatic lifting operation for the fork and tilting angle control of the upright. An input unit of the microcomputer system comprises a plurality of sensors to be described hereinbelow and microcomputer input interface circuit connected to the sensors. One of the sensors includes a first sensor provided for detecting the height of the fork lifted upward from its lowest position. The first sensor comprises a disc having a plurality of slits along the radial direction thereof and a photocoupler provided across the disc so that the disc can rotate together with the chain wheel through the photocoupler. It will be noted that the disc is attached coaxially to the chain wheel. The photocoupler comprises a light emitting member, e.g., LED (light emitting diode) which emits light toward the disc and a light receiving member, e.g., photo transistor which receives the light passed through the slits provided through the disc and converts the received light into an electrical signal. If the number of the pulse-shaped signals electrically converted on a basis of the light passed through the slits of the disc is counted by means of a counter, the microcomputer can determine the height of the fork lifted from the lowest position. Other sensors include a second sensor for detecting a tilting angle of the upright and third sensor for detecting the presence of load on the fork. The second sensor comprises a potentiometer, located adjacent to the tilt cylinder, across which a DC voltage is applied. The potentiometer is provided with an operation lever and pin attached to the top end of the operation lever, the pin inserted into an elongated hole provided at an oblique angle within a fixed member attached around the outer surface of the piston of the tilt cylinder, so that the operation lever rotates clockwise or counterclockwise as the piston of the tilt cylinder pushes or pulls the outer mast to adjust the tilting angle of the upright. Consequently, the potentiometer sends a variable voltage signal to the input interface circuit of the microcomputer system.
The input interface circuit for the second sensor comprises an analog-to-digital converter. The analog-to-digital converter used in this fork lift truck control device comprises nine resistors, five variable resistors, and five comparators. That is to say, all noninverting input terminals of the five comparators are connected to the potentiometer, i.e., second sensor via a resitor and each inverting input terminal of the five comparators is connected to one of a plurality reference voltage sources formed with a DC voltage supply, resistors, and variable resistors for parallel comparison of the received analog voltage from the potentiometer with each reference voltage corresponding to a value of the tilting angle of the upright. Therefore, a first comparator has a first reference voltage at its inverting input terminal, a second comparator has a second reference voltage at its inverting input terminal, a third comparator has a third reference voltage at its inverting input terminal, a fourth comparator has a fourth reference voltage at its inverting input terminal, and a fifth comparator has a fifth reference voltage at its inverting input terminal. All comparators are previously adjusted to provide a logical "0" level signal when zero voltage or voltage below respective reference voltages is applied to the noninverting input terminals of the comparators. The first reference voltage corresponds to a zero degree (neutral position) tilting angle of the upright, the second reference voltage corresponds to one degree of the backward tilting angle of the upright, the third reference voltage corresponds to three degrees of the backward tilting angle of the upright, the fourth reference voltage corresponds to four degrees of the backward tilting angle of the upright, and the fifth reference voltage corresponds to twelve degrees of the backward tilting angle of the upright.
Therefore, e.g., when the backward tilting angle of the upright is between zero degree and one degree, the output bit string of the analog-to-digital converter indicates 00001 and when the backward tilting angle of the upright is more than twelve degrees, the output bit string of the analog-to-digital converter indicates 11111. It will be noted that the analog-to-digital converter of the type described above is not provided with an encoder circuit for weighing each bit signal since each meaning of the output bit strings is previously identified by the microcomputer main frame.
The third sensor is provided for detecting the weight of load applied on the fork for changing a target value of the tilting angle of the upright so as to place the fork in a horizontal position with respect to the truck body due to the bending of the upright and fork which vary depending on the weight of load, e.g., by measuring a hydraulic pressure within the lift cylinder or by measuring both hydraulic pressure and pneumatic pressure of a front wheel of the vehicle body. When a drive signal indicating that a load is piled on the fork is sent from the third sensor into a switch constituting the input interface circuit to drive the switch to close, the switch is closed to send a "1" signal into the microcomputer main frame. When no drive signal is sent from the third sensor into the switch, the switch remains off so that the microcomputer receives a "0" signal from the switch and judges that no load is piled on the fork.
On the other hand, when the potentiometer output voltage exceeds the first reference voltage, the first comparator only sends a "1" bit signal via a first signal line D.sub.A of five parallel signal lines D.sub.A through D.sub.E within the common bus into the microcomputer main frame to indicate that the upright is tilted more than zero degree (0.degree.) backward (toward the truck body) with respect to the upright position vertically disposed to the ground. Similarly, when the output voltage of the potentiometer exceeds the second reference voltage, the first and second comparators send a 37 1" bit signal through the first and second signal lines into the microcomputer main frame to indicate that the angle of the upright is more than one degree (1.degree.). Consequently, when the output voltage of the potentiometer exceeds the fifth reference voltage, all comparators send "1" bit signals through the signal lines into the microcomputer main frame to indicate that the angle of the upright is tilted backward more than twelve degrees (12.degree.).
Hence, the microcomputer judges from the "0" level signal received from the switch that no load is applied on the fork and performs a feedback control over the tilt cylinder so that the upright tilts backward to an angle within unloaded neutral range {zero degree (0.degree.) through one degree (1.degree.) or, the bit string of the A/D converter indicates 00001}. That is, an operational command is given to a sevomotor circuit connected to the microcomputer main frame to actuate a hydraulic pressure control valve so that the tilt cylinder is operated to tilt the upright at the target value described hereinabove. On the other hand, the microcomputer judges from the "1" level signal received from the switch that a load is applied on the fork and performs the feedback control over the tilt cylinder so that the upright tilts backward at an angle within loaded neutral range (3.degree. through 4.degree., i.e., the bit string of the A/D converter indicates 00111).
There is, however, a drawback in the input interface circuit of the microcomputer, i.e., a counting circuit for informing the microcomputer main frame of the lifting height of the fork on a basis of the output signal from the first sensor and the analog-to-digital converter for outputting a bit string corresponding to the upright tilting angle. In more detail, in the case of the lifting height counter, since the pulse-shaped signal from the photocoupler, i.e., first sensor is compared directly with a reference voltage subsequently to a waveform shaper so that a rectangular wave is formed and sent into an up/down counter, the amplitude of the pulse-shaped signal generated from the photocoupler is not sufficiently large and stable to enable direct comparison with the reference voltage. In addition, such counting circuit cannot follow the repeated upward and downward movements of the fork within a short distance. Consequently, it is difficult for the UP/DOWN counter to count correctly the number of rectangular pulses produced on a basis of the output signal of the photocoupler.